Repair mortar for a coloured layer of sgraffito render – a technological copy

0561611 - ÚTAM 2023 RIV SI eng C - Conference Paper (international conference)
Válek, Jan - Skružná, Olga - Wichterlová, Z. - Waisserová, J. - Kozlovcev, Petr - Frankeová, Dita
Repair mortar for a coloured layer of sgraffito render – a technological copy.
Proceedings of the 6th Historic Mortars Conference - HMC 2022. Ljubljana: University of Ljubljana, 2022 - (Bokan Bosiljkov, V.; Padovnik, A.; Turk, T.; Štukovnik, P.), s. 235-248. ISBN 978-961-6884-77-8.
[Historic mortars conference HMC 2022 /6./. Lublaň (SI), 21.09.2022-23.09.2022]
R&D Projects: GA MK(CZ) DG20P02OVV028
Keywords : sgraffito render * Slavonice * replication
OECD category: Materials engineering

Sgraffito technique was used to decorate renders by scratching the top layer of lime wash in the Renaissance time. This technique required both artistic and craft skills and its quality and durability relied on the selected materials. In order to contribute to the preservation of surviving sgraffiti in the town of Slavonice in the Czech Republic a study was carried out assessing the possibility to replicate the original materials and the application techniques. Historical sgraffito layers were sampled in situ and studied in a laboratory by commonly used analytical methods - OM, TA, XRD, SEMEDS. The raw materials, lime binder and sand, were characterised and the mixing proportion app. 1 to 0.7 (vol.) of lime putty to sand was determined. The character of the raw materials was compared with the locally known resources and their probable provenance was localised. Based on the character of the local limestone, a similar raw material was obtained and burnt in an experimental lime kiln to produce quicklime. The sand was obtained locally from an old and disused pit quarry. The possibility to obtain the raw materials that came from similar sources as the historic ones allowed us to study possible production technologies and application techniques. The ways the raw materials were processed and the mortar applied were verified by a series of practical experiments. These included the use of lime putty v. dry slaked hydrate, the thickness of a layer, trowelling and final finishing, time span before application of lime wash, timing of drawing and scratching. In addition, the performance of the produced mortar mix was assessed by mechanical and physical tests. Compressive and flexural strengths, capillary absorption, drying index, open porosity and water vapour diffusion coefficient were determined on standard prism specimens as well as on mortar specimens cut from wall panels. The mortar was tested in two sets with different consistencies due to mixing water content. The tests pointed out the influence of application methods on the performance. The mortar properties were positively evaluated especially in terms of porous structure and water vapour permeability. The mortar, designed as a material replica of the original, was used in a conservation project on a façade
of a house, where missing parts of a sgraffito render were reconstructed.
Permanent Link: https://hdl.handle.net/11104/0334297